1. Field of the Invention
The present invention relates to an adhesive sheet, especially to an adhesive sheet which is suitable for protecting a semiconductor circuit when a semiconductor wafer, upon which high density circuit patterns are mounted, is processed.
2. Description of the Related Art
A backside surface of a semiconductor wafer is ground after circuits are formed on a front side surface thereof, so that the thickness of the semiconductor wafer is adjusted. During the grinding process, a protection sheet including an adhesive sheet is adhered to the front side surface to protect the circuits mounted thereon. Such a protection sheet is required not only to prevent damage to the circuits or the wafer body, but also to prevent contamination to the circuit caused by residual adhesive matter following removal, to prevent the penetration of water, for washing off ground waste and cooling tools, onto the circuit surface, and to contribute to the accuracy of the wafer thickness resulting from the grinding process. As such a protection sheet, an adhesive sheet including an ultraviolet ray curable adhesive, is known (Japanese unexamined Patent Publication No. S60-189938).
In regular manufacturing processes, a semiconductor wafer is chipped by a dicing process after a grinding process. Recently, treating a ground wafer has become increasingly difficult in semiconductor manufacturing processes, because the diameter of the wafer has been increasing while the thickness of the wafer has been decreasing, thus the semiconductor wafer is becoming increasingly breakable. Therefore, using a DBG process (that is, dicing before grinding process), where the wafer is partially cut (i.e. the half-cut process) before the grinding process chips the wafer, is promising. In a DBG process, a protection sheet is adhered to the circuit surface of a wafer after undergoing the half cut process (Japanese unexamined Patent Publication No. H05-335411).
In a conventional process, a protection sheet adhered to the circuit surface of a wafer need only prevent penetration of water at the edge of the wafer. On the contrary, in a DBG process, a sufficient adhesion to the surface of each chip of a wafer is required to prevent the penetration of the washing water, because the wafer has already been chipped during the grinding process. When the adhesion of a protection sheet is increased to be firmly adhered to the circuit surface of the wafer, it increases the problem of adhesion residue remaining on the circuit surface after the protection sheet has been stripped away. To solve this problem, an adhesive sheet including an ultraviolet ray curable adhesion has been used as a protection sheet has been used, for example (Japanese unexamined Patent Publication No. 2000-68237).
In one step of the DBG process that differs from a conventional process, following the division of the wafer into a plurality of small chips, the distance separating the individual chips has a tendency to deviate unacceptably (kerf shift) due to the effects of pressurization on each chip during a grinding operation. The breakdown in the alignment of the chips, as explained above, is problematic because the chips may come into contact with each other in the grinding, delivering, or pick-up process.
Because the shapes of semiconductor parts have been changing with respect to the past, relatively uneven elements such as an electrode tend to collect at the periphery of a semiconductor chip, that is, uneven elements tend to be concentrated in a narrow area. Therefore, effectively adhering a protection sheet to the edge of a semiconductor chip is becoming more difficult, so that the protection sheet used in the DBG process may not seal the circuit surface effectively due to poor adhesion to the circuits (followability to bond to the uneven circuit surface). As a result, a problem where water for grinding penetrates the circuit surface has arose. Further, if improving the followability to bond to the uneven circuit surface of the protection sheet, another problem where the cohesion of the adhesive is lowered, so that the kerf shift frequently happens, has arose.